1. Field of the Invention
An object of the present invention is to provide an integrated circuit having a circuit for detecting an operating voltage, the circuit being useful in the field of semiconductor circuits, notably CMOS type semiconductor circuits.
2. Discussion of the Related Art
There are known circuits for detecting the crossing, by a voltage to be monitored, of a reference voltage. These circuits typically have differential amplifiers for receiving a reference voltage and an operating voltage. Such circuits require a precise external reference voltage. These circuits have an advantage in that the reference voltage may have any value since it is fixed externally. One of the problems to be resolved, however, is in producing a reference voltage whose value can be set as desired but which, once chosen, is the same for all the fabricated circuits.
In the manufacture of integrated circuits, variations are found in the characteristics of the circuits because of the method of manufacturing the circuit, and because of the conditions of use. The manufacturing variations reflect variations undergone by the circuit during the different stages of its manufacture. Many parameters come into play in a manufacturing process, including the duration and density of ion implantations, and the nature and quality of masking operations. The manufactured product is highly sensitive to these manufacturing parameters and thus, there is a great variation in the characteristics obtained. During use, an integrated circuit is subjected to stresses, notably relating to temperature and supply voltage, which may have harmful repercussions on the chosen threshold voltages.
Another device for detecting of threshold voltages has been proposed. It uses the avalanche voltage of a diode, preferably a Zener diode. The detected threshold voltage of a Zener diode manufactured according to a given method will vary as a function of the temperature of use of the circuit only from, for example, 5.35 to 5.48 volts. This low variation is a valuable feature. The reference voltage thus obtained has a drawback, however, in that, for a given method, it cannot be set as desired since it is determined by the given manufacturing method.
This situation shows that known circuits for delivering reference voltages which are used to compare an operating voltage with a reference voltage are either complicated, stable, constant, and adjustable; or simple, stable, and constant, but not adjustable.